A method of this type is known from Patent Application No. GB PA 2.241.185. According to the known method, there are applied, as a first and second layer, a layer of copper on a metal strip. The strip is then rolled to form a multiple-walled tube. According to the known method, the strip is rolled through two complete revolutions, thus forming a double-walled tube. The fact that the tube has double walls implies that, between the two walls of the tube, there is also a layer of copper. After rolling of the strip, the tube thus formed is heated in order to subject to brazing the surfaces of the walls which are in contact with one another.
The application of a layer of copper or of another brazeable metal to one or both sides of a metal strip has the advantage of improving the technical qualities of the tube, particularly as regards corrosion resistance by application of a layer of nickel, its suitability for brazing or its protection from the liquids circulating in the tube.
Whereas the application of a layer of metal to the strip does in fact offer advantages, it has however been noted that it could likewise give rise to problems. For example, in the case of tubes used as brake-fluid lines in a vehicle, the copper layer inside the tube offers good resistance to brake fluid, an aggressive substance, but the external copper layer does not offer sufficient corrosion protection for the tube, which is sited in places extremely exposed to bad weather. The tube must then be protected by an additional covering, for example of zinc. However, the copper layer, already applied to the metal strip, is not ideal as regards electrochemical couple and limits the quality of the whole product as regards corrosion.
Another problem noted is the dissolution of copper applied to the internal surface of the tube. Certain alcohols used as fuel additives, particularly in lead-free petrol, attack and dissolve the copper, which finally blocks the injectors of combustion engines.